Carbon Nanotubes (CNTs) have many exceptional properties that make them attractive for a variety of applications. In particular, past works have shown that CNTs can have outstanding electrical field emission properties, with high emission currents at low electric field strengths (applied field as low as 1-3 V/μm and emission current as high as 0.1 mA from a single nanotube). Carbon nanotubes are therefore attractive as cold-cathode field emission sources, especially for applications requiring high current densities (hundreds to thousands of amperes per cm2) and lightweight packages (high frequency vacuum tube sources). However, it is also well known that the high emission capability of a single nanotube does not necessarily translate directly into high emission magnitudes from a larger area sample containing many such nanotubes because of the electrostatic screening effect.
A number of different field emitter designs have been proposed including isolated nanotubes and dense, continuous mats of nanotubes. No agreement has been reached on the optimum geometry for producing high current densities in these field emitters. For example, while Nilsson et al have provided simulations showing that the optimum nanotube packing density with best field penetration occurs when the inter-tube spacing is at least twice that of the nanotube height (Nilsson, et al., Appl. Phys. Lett. 2000, 76, 2071-2073), Suh et al have performed measurements that appear to shown that it is when the inter-tube spacing is equal to the nanotube height. (Suh, et al., Appl. Phys. Lett. 2002, 80, 2392-2394.) More recently, there have been many fundamental works on field emission optimization from CNTs using sparse, dense and patterned arrays of either forests or individual, vertically-aligned nanotubes or nanofibers. (See, e.g., Merkulov, et al., Appl. Phys. Lett. 2001, 89, 1933-1937; Chowalla, et al., Appl. Phys. Lett. 2001, 79, 2079-2001; Semet, et al. Appl. Phys. Lett. 2002, 81, 343-345; Jo, et al., Appl. Phys. Lett. 2003, 82, 3520-3522; and Teo, et al., Appl. Phys. Lett. 2002, 80, 2011-2013, the disclosure of which are incorporated herein by reference.) But, achieving high current densities (hundreds to thousands of amperes per square centimeter) over large nanotube sample areas with repeatability and emission longevity still remains an open problem. Indeed, despite the intensive research efforts current carbon nanotube field emitter have only reached current densities from CNT of 100-500 mA/cm2 over areas on the order of 100 μm×100 μm. (See, e.g., Thong, et al., Appl. Phys. Lett. 2001, 79, 2811-2813; Sohn, et al., Appl. Phys. Lett. 2001, 78, 901-903; Rao, et al., S. Appl. Phys. Lett. 2000, 76, 3813-3815; and Fan, et al., H. Science, 1999, 283, 512-514, the disclosures of which are incorporated herein by reference.)
Accordingly, a need exists for improved carbon nanotube field emitters capable of achieving higher current densities.